The contribution of each leg to bipedal balance control

Maintaining balance is a task that healthy adult humans perform effortlessly throughout the day. However, when people age, or when they develop a neurological disease (e.g. Parkinson’s disease; PD) the risk of a fall increases, showing the complexities of maintaining balance. In this thesis, I investigated how both legs work together to maintain balance, both in healthy subjects and in people with PD. Chapter 1 provides a general introduction and introduces the research questions. Chapter 2 reviews the state of the art regarding gait and balance research in PD. It confirmed the high risk of falling in PD patients and it suggested that this could be due to asymmetries in motor performance, impaired integration of sensory signals and a difficulty of switching between sensory modalities. Chapter 3 describes a newly developed method to identify the balance control contribution of the hip and ankle joints of both legs separately, using closed‐loop system identification methods. Two separate mechanical perturbations were applied and the participants’ responses were determined with a dual forceplate and motion capture. The method was successfully applied in healthy subjects and a PD patient and showed that multisegmental balance control can be reliably identified using two independent perturbations. Chapters 4, 5 and 6 focused on the balance control contribution of each leg in PD patients. It was established that balance control can be asymmetrical in PD patients. However, it seemed that asymmetrical balance control was not related to clinical outcome measures such as freezing of gait. Interestingly, it was shown that PD patients can compensate for their most affected leg, by increasing the balance control contribution of the least affected leg. This could paradoxically increase the balance asymmetry. Chapter 7 investigated whether healthy subjects are capable to suppress erroneous sensory information of each leg separately. To this end, the balance of healthy volunteers was perturbed by rotating the support surface of each foot through the ankle joint axis. The results showed that subjects suppressed the sensory information of the most perturbed leg, independent of the other leg. Chapter 8 discusses the general findings of this thesis.